SSIS Architecture and Internals Interview Questions
When preparing for SSIS interview it is important to understand what questions could be asked in the interview. In this tip series (tip 1, tip 2, tip 3), I will try to cover common SSIS interview questions to help you prepare for a future SSIS interview. In this tip we will cover interview questions related to the SSIS architecture and internals. Check it out.
What are the different components in the SSIS architecture?
- The SSIS architecture comprises of four main components:
- The SSIS runtime engine manages the workflow of the package
- The data flow pipeline engine manages the flow of data from source to destination and in-memory transformations
- The SSIS object model is used for programmatically creating, managing and monitoring SSIS packages
- The SSIS windows service allows managing and monitoring packages
- To learn more about the architecture click here.
How is SSIS runtime engine different from the SSIS dataflow pipeline engine?
- The SSIS Runtime Engine manages the workflow of the packages during runtime, which means its role is to execute the tasks in a defined sequence. As you know, you can define the sequence using precedence constraints. This engine is also responsible for providing support for event logging, breakpoints in the BIDS designer, package configuration, transactions and connections. The SSIS Runtime engine has been designed to support concurrent/parallel execution of tasks in the package.
- The Dataflow Pipeline Engine is responsible for executing the data flow tasks of the package. It creates a dataflow pipeline by allocating in-memory structure for storing data in-transit. This means, the engine pulls data from source, stores it in memory, executes the required transformation in the data stored in memory and finally loads the data to the destination. Like the SSIS runtime engine, the Dataflow pipeline has been designed to do its work in parallel by creating multiple threads and enabling them to run multiple execution trees/units in parallel.
How is a synchronous (non-blocking) transformation different from an asynchronous (blocking) transformation in SQL Server Integration Services?
- A transformation changes the data in the required format before loading it to the destination or passing the data down the path. The transformation can be categorized in Synchronous and Asynchronous transformation.
- A transformation is called synchronous when it processes each incoming row (modify the data in required format in place only so that the layout of the result-set remains same) and passes them down the hierarchy/path. It means, output rows are synchronous with the input rows (1:1 relationship between input and output rows) and hence it uses the same allocated buffer set/memory and does not require additional memory. Please note, these kinds of transformations have lower memory requirements as they work on a row-by-row basis (and hence run quite faster) and do not block the data flow in the pipeline. Some of the examples are : Lookup, Derived Columns, Data Conversion, Copy column, Multicast, Row count transformations, etc.
- A transformation is called Asynchronous when it requires all incoming rows to be stored locally in the memory before it can start producing output rows. For example, with an Aggregate Transformation, it requires all the rows to be loaded and stored in memory before it can aggregate and produce the output rows. This way you can see input rows are not in sync with output rows and more memory is required to store the whole set of data (no memory reuse) for both the data input and output. These kind of transformations have higher memory requirements (and there are high chances of buffer spooling to disk if insufficient memory is available) and generally runs slower. The asynchronous transformations are also called "blocking transformations" because of its nature of blocking the output rows unless all input rows are read into memory. To learn more about it click here.
What is the difference between a partially blocking transformation versus a fully blocking transformation in SQL Server Integration Services?
- Asynchronous transformations, as discussed in last question, can be further divided in two categories depending on their blocking behavior:
- Partially Blocking Transformations do not block the output until a full read of the inputs occur. However, they require new buffers/memory to be allocated to store the newly created result-set because the output from these kind of transformations differs from the input set. For example, Merge Join transformation joins two sorted inputs and produces a merged output. In this case if you notice, the data flow pipeline engine creates two input sets of memory, but the merged output from the transformation requires another set of output buffers as structure of the output rows which are different from the input rows. It means the memory requirement for this type of transformations is higher than synchronous transformations where the transformation is completed in place.
- Full Blocking Transformations, apart from requiring an additional set of output buffers, also blocks the output completely unless the whole input set is read. For example, the Sort Transformation requires all input rows to be available before it can start sorting and pass down the rows to the output path. These kind of transformations are most expensive and should be used only as needed. For example, if you can get sorted data from the source system, use that logic instead of using a Sort transformation to sort the data in transit/memory. To learn more about it click here.
What is an SSIS execution tree and how can I analyze the execution trees of a data flow task?
- The work to be done in the data flow task is divided into multiple chunks, which are called execution units, by the dataflow pipeline engine. Each represents a group of transformations. The individual execution unit is called an execution tree, which can be executed by separate thread along with other execution trees in a parallel manner. The memory structure is also called a data buffer, which gets created by the data flow pipeline engine and has the scope of each individual execution tree. An execution tree normally starts at either the source or an asynchronous transformation and ends at the first asynchronous transformation or a destination. During execution of the execution tree, the source reads the data, then stores the data to a buffer, executes the transformation in the buffer and passes the buffer to the next execution tree in the path by passing the pointers to the buffers. To learn more about it click here.
- To see how many execution trees are getting created and how many rows are getting stored in each buffer for a individual data flow task, you can enable logging of these events of data flow task: PipelineExecutionTrees, PipelineComponentTime, PipelineInitialization, BufferSizeTunning, etc. To learn more about events that can be logged click here.
How can an SSIS package be scheduled to execute at a defined time or at a defined interval per day?
- You can configure a SQL Server Agent Job with a job step type of SQL Server Integration Services Package, the job invokes the dtexec command line utility internally to execute the package. You can run the job (and in turn the SSIS package) on demand or you can create a schedule for a one time need or on a reoccurring basis. Refer to this tip to learn more about it.
What is an SSIS Proxy account and why would you create it?
- When we try to execute an SSIS package from a SQL Server Agent Job it fails with the message "Non-SysAdmins have been denied permission to run DTS Execution job steps without a proxy account". This error message is generated if the account under which SQL Server Agent Service is running and the job owner is not a sysadmin on the instance or the job step is not set to run under a proxy account associated with the SSIS subsystem. Refer to this tip to learn more about it.
How can you configure your SSIS package to run in 32-bit mode on 64-bit machine when using some data providers which are not available on the 64-bit platform?
- In order to run an SSIS package in 32-bit mode the SSIS project property Run64BitRuntime needs to be set to "False". The default configuration for this property is "True". This configuration is an instruction to load the 32-bit runtime environment rather than 64-bit, and your packages will still run without any additional changes. The property can be found under SSIS Project Property Pages -> Configuration Properties -> Debugging.
- Review the following resources for your next SQL Server Integration Services interview:
- Check out the previous tips in this series:
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Article Last Updated: 2012-01-02